The cellulose industry generates spent liquors differing in composition according to the delignification process used. Within the sulphate pulping industry, spent liquor, commonly referred to as black liquor, contains valuable chemicals and energy in the form of combustible carbonaceous compounds. At the present time these chemicals and energy are normally recovered in a recovery boiler in which the black liquor is completely burned.
Partial combustion of black liquor in a gasification reactor as in the present invention generates a combustible gas comprising H.sub.2, CO, CO.sub.2, and droplets of molten inorganic chemicals.
In conjunction with pulp bleaching, a diluted liquor comprising organic matter and sodium salts is obtained. Mechanical and semi-chemical pulping processes also generates diluted liquors of different compositions. These as well as other waste and spent liquors generated in the cellulose industry can, after concentration, be used as a feedstock in the process of the present invention.
Although the following description describes the present invention as it applies to black liquor it is not restricted only to this particular liquor in its application.
The mechanisms related to partial combustion of black liquor are fairly well understood and are applied inter alia in the lower part of the soda recovery boiler. The difference between the present burner and a liquor burner in a soda recovery boiler is, however, great inter alia due to the low degree of liquor atomization in recovery boiler burners and the absence of a well-defined liquor flame. Another important difference between a recovery boiler burner and the burner of the present invention is that the present burner is primarily intended for gasification at elevated pressures.
A major difference between the burner of the present invention and conventional oil burners is that a stable flame has to be formed with the use of a considerably lower amount of air or oxygen carrier.
As the exemplification below shows black liquor as a fuel is characterized by a relatively low calorific value and high water and ash contents.
______________________________________ Calorific value of 13 GJ/ton dry substance (DS) the dry substance Elementary composition C.sub.29 H.sub.34 O.sub.20 Na.sub.9 S.sub.2 Dry solids content 65% Viscosity at 100.degree. C. 100 cSt. ______________________________________
The presence of sodium compounds in the black liquor and its inherently high oxygen content make it a very reactive fuel, which means, provided an adequate burner is at hand, that the carbon conversion already in the flame zone becomes high, in spite of the fact that the combustion is substoichiometric.
The degree of atomization of the liquor is of great importance for obtaining a stable black liquor flame, the extension of the flame and high carbon conversion. The rheological properties of the black liquor are of significant importance to the degree of atomization which can be achieved in a given nozzle. The viscosity of the black liquor can be influenced by e.g. heating and/or the addition of additives. Normally the black liquor is being heated to above 100.degree. C. for use in the present invention. The viscosity of the black liquor at the moment of atomization should preferably be below 200 cSt.
Atomization of the black liquor can be further enhanced by flashing the liquor into the reactor in which case the liquor is preheated to a temperature above its boiling point at the operating pressure of the reactor.
Several types of atomizing nozzles are available but only a few varieties are suitable for atomizing cellulose spent liquors, such as black liquor, in the present invention.
"Twin-fluid" nozzles are most suitable for use in the present burner. A common feature of "twin-fluid" nozzles is that a relatively high gas flow rate is necessary for the supply of energy for the atomization. Another important feature of these nozzles is that the resulting size of the droplets decrease with increasing density of the atomizing gas. Depending on how the two fluid phases are brought together several mechanisms for forming droplets, such as shearing between ligaments, combination and formation of spheres of liquor droplets and high turbulence decomposition of the liquor spray can be anticipated.